The present invention relates to a system and method for qualitatively/quantitative detection of an analyte in a sample and more particularly, a system using an implantable sensor beneath the skin and a portable detection device that can take measurements non-invasively.
Various devices and methods have been provided for monitoring concentrations or levels of analytes in humans and other animals to determine certain conditions of a subject, including analysis of diseases present. Present devices and methods used to monitor many conditions, however, are intrusive and may have undesirable or deleterious effects. For example, persons with diabetes may need to continuously monitor their glucose levels. Typically, a patient draws and analyzes glucose levels in his/her own blood multiple (6-8) times daily to accurately control their glucose level. A substantial proportion of diabetics do not perform this task because with available techniques it may be awkward, time consuming, and sometimes painful.
Rather than drawing blood with a needle and measuring the glucose levels externally, other methods have focused on in situ analysis of the glucose levels in subcutaneous fluid, which correlates with the concentration of the glucose in the bloodstream. One of the methods to measure the glucose levels requires puncturing the dermal tissue and inserting an optical fiber, which has a structure on the end embedded in the subcutaneous fluid. The structure houses chemicals that fluorescence when activated by glucose molecules and may have a mirror on the end of the structure opposing the end of the optical fiber. The end of the fiber is attached to a photomultiplier tube, which detects the amount of fluorescence. However, this type of system is inefficient, large and bulky, and the optical fiber through the skin may pose risk.
In an attempt to avoid the problems associated with an optical fiber puncturing and held through the skin, implantable optical analyte-sensors have recently been developed. These sensors are implanted under the dermal tissue and placed in the subcutaneous fluid during an operation. The sensors are optically interrogated, thereby preventing the need to penetrate the skin to determine glucose levels.
However, optically interrogated sensors have proven unworkable over extended use time periods due in part to mechanical reasons, such as leakage at the assembly seams, and chemical reasons, such as photobleaching of the fluorescent labels present inside the sensor, which require frequent calibration.
However, numerous other problems exist for systems using the conventional implanted sensors. For example, in general the excitation-source and detector remain full sized (such as a packaged laser and photomultiplier tube) and thus remote to the sensor. Furthermore, because both the excitation-source and detector are remote to the sensor, optical fibers are used, in this case to transmit the excitation light from the excitation-source to the sensor and response light from the sensor to the detector. This system is thus both cumbersome and expensive.
In view of the above, a portable device that monitors analyte levels and provides an indication of the level, is relatively small, and can be used to take readings in a non-invasive manner is provided by using a combination of a portable instrument with a novel configuration and a novel reference source. The system also provides compensation for photodrift and other conventional problems thereby extending the interval between any necessary calibration. Similarly, a method for monitoring analyte levels and providing an indication of the level is provided.
In one embodiment, the optical irradiation system includes a sensor disposed beneath a user""s dermal tissue that contains a fluorescently labeled analogue, and a reference. The labeled analogue and reference are in close proximity with each other. The labeled analogue emits a first electromagnetic signal and the reference emits a second electromagnetic signal having a different wavelength than the first electromagnetic signal. The fluorescent intensity of the labeled analogue is proportional to the concentration of the analyte of interest. By measuring the intensity of the labeled analogue, the concentration of analyte may be calculated. The optical irradiation system also includes a portable instrument that is used external to the dermal tissue. When a reading is taken, the portable instrument is placed above and adjacent to the sensor.
The portable instrument includes an optical excitation-source, for example a laser diode, which generates radiation and irradiates the sensor through the dermal tissue. A receiver in the portable instrument detects and discriminates between the two electromagnetic signals and produces a first electrical signal corresponding to the first electromagnetic signal and a second electrical signal corresponding to the second electromagnetic signal. Logic in the portable instrument receives and processes the first and second electrical signals and generates an indication of the detected analyte level.
The portable instrument can be placed on or adjacent to the user""s skin. One benefit of placing a reference in close proximity to the labeled analogue is the ability to maximize the emission intensity available from the labeled analogue emitter by maximizing the emission intensity from the reference. When the analyte sensing device is implanted, and the excitation-source or laser is above the skin, the invariant (analyte independent) nature of the reference emission may be used to fine tune the position of the external device in relation to the implant for maximum signal.
In another aspect of the invention, the configuration of an optical element in the portable instrument is such that the optical element increases the light gathering ability of the device, and directs the radiation from the sensor to substantially impinge on the receiver at an angle less than about 20xc2x0 from normal, so that full advantage can be taken of the interference filters used to discriminate between the two electromagnetic signals.
The following figures and detailed description of the preferred embodiments will more clearly demonstrate these and other advantages of the invention.